Matching circuits are widely used to transform the impedance of various components within a circuit either to a target reference impedance (e.g. a transmission line impedance and/or test port) or to directly match two components with different (possibly complex) impedances for optimum power transfer. A matching circuit typically has two nodes for insertion in-line between the components or between a component and a reference impedance. Both nodes of the matching circuit may be carrying a signal in either or both directions through the matching component. The matching function is particularly advantageous for signals at radio frequencies. The matching circuit may precede or follow a component, which has particular input and output impedances when embedded in the circuit. If the component follows the matching circuit in the signal path, the target impedance of the component is its input impedance. If the component precedes the matching circuit in the signal path, the target impedance of the component is its output impedance.
The matching circuit functions to set the impedance seen by the signal to the target impedance by compensating for the difference between the impedance of interest and the target impedance. The compensation of the impedance is determined by the configuration of the matching circuit and the component values of the matching circuit. A wide range of impedance matching and transfer function circuits can be realized by using lumped element inductors or capacitors or both. At higher frequency (above about 1 GHz), it is often advantageous to replace either lumped element inductors or capacitors or both with distributed transmission line networks. Even lumped elements may take on transmission line characteristics at these high frequencies. The usefulness of this replacement is also dependent on the dielectric constant (DK) of the substrate, as well as area constraints.
Different arrangements of matching circuits are known in the art. The selection of the circuit depends on the impedances to be matched. Some examples include series capacitor, shunt capacitor; series capacitor, shunt inductor; series inductor, shunt capacitor; etc. A common configuration for a matching circuit is a sequence of shunt capacitor, series inductor and shunt capacitor known as a pi-network. Transformers, and even resistive networks, can be used, if the insertion loss can be tolerated.
In general, matching circuits should have minimum loss to prevent added degradation in the information signal. Excess loss increases the demands made on other components in an electronic system, especially the active elements such as amplifiers. At the input to a low noise amplifier (LNA), increased signal loss cannot be made up (compensated for) by simply increasing gain of the LNA due to noise considerations. Similarly, signal loss at the output of a power amplifier increases the power consumption of the amplifier to achieve a given output power.
The impedances of the components and the matching circuits are frequency dependent. The impedance can only be perfectly matched at a single operating frequency or optimally matched over a limited band of frequencies. If the designer wishes to operate the device at more than one frequency band, compromises must be made in performance or in circuit complexity. Often, separate signal paths and circuits are used for different frequency bands to enable separate performance optimization. This adds to the cost and size of the circuit and requires of use of signal selection circuitry such as switches or diplexers.
Tunable impedance matching networks can provide an advantage over fixed impedance matching networks. In particular, tunable impedance matching networks can include controllable elements that can be optimally tuned for desired frequencies. Further, tunable impedance matching networks provide an advantage of being able to adapt to environmental and component variations. For example, antenna impedance can vary when objects are positioned near the antenna. Further, impedance can vary based on temperature and based on component manufacture.
In wireless handsets, tunable matching circuits at frequencies above about 200 MHz have proven difficult to achieve. It will be appreciated that a low loss tunable matching network or a tunable diplexer matching system would be beneficial in many applications, but particularly in a portable wireless communication device.